It has long been recognized that normal skin exhibits a substantial level of fluorescence(Fellner, Arch. Dermatol. 112: 667-670, 1976). The fluorescence apparently exists throughout the different layers of the skin, with the epidermis showing the weakest levels, the stratum corneum being slightly stronger, and the most intense emissions being found in the dermis and subcutaneous fat(Zeng, et al., Photochem. Photobiol. 61: 639-645, 1995). The level of epidermal fluorescence varies depending upon the color of the individual's skin, with darker skins showing a higher level of fluorescence than lighter skins. However, the fluorescence in the dermis is apparently related to elements common to all skin types: elastin and collagen. The spectra of living human skin is measurable over a wide excitation wavelength, with green being the dominant autofluorescence color.
With particular respect to the dermis, it well-known that the elements responsible for fluorescence are susceptible to substantial alteration in quality and quantity due to advancing age as well as UV exposure. It is widely accepted that these changes in elastin and collagen are at least partially, and probably predominantly, responsible for many of the external changes characteristic of aged skin, whether chrono- or photoaged. The external changes that are immediately identifiable as being associated with loss or alteration of these fibers include the readily defined features, such as lines, wrinkles, and skin atrophy; however, another common age-associated feature that is perhaps more difficult to characterize is familiar loss of luster, color and tone of mature or photodamaged skin.
Interestingly, the change in structure of collagen and elastin observed at least with respect to photoaging has been shown to be correlated with a decline in the intensity of fluorescence in the photoaged skin.(Leffell, et al. Arch. Dermatol. 124: 1514-1518, 1988). This change is also reflected in chronoaged skin, which in middle age begins to lose its green fluorescence, and in later years, loses its blue fluorescence. It is very likely that the decline in the vigorous "glow" common to young, healthy skin is related at least in part to the this observed loss of fluorescence. Nonetheless, cosmetics and skin care products have traditionally focused on the camouflaging of the most easily characterized signs of aging, such as wrinkles; there has been little effort to develop products which address the seemingly more intangible problem of renewing the glow of youth in the more mature individual's skin. The present invention now provides a solution to this problem.